1. Field of the Invention
The present invention relates to a lubricating structure for a hydraulic driving apparatus, for driving a drive shaft under a pressing action effected using an operation oil.
2. Description of the Related Art
A hydraulic driving apparatus, which is driven by the aid of a pressure fluid (for example, pressure oil), has been hitherto used, for example, in order to transport a workpiece and/or for positioning the workpiece.
For example, in the case that a hydraulic actuator is used as a hydraulic driving apparatus, a piston, which is displaceable in an axial direction, is provided in a cylindrical main cylinder body, the main cylinder body having both ends closed by a pair of closing members. Pressurizing chambers are provided on respective end sides of the piston. Operation oil is supplied into the pressurizing chambers by the aid of a hydraulic connecting member disposed outside of the main cylinder body. The piston makes rectilinear motion in the axial direction under the pressing action effected by the operation oil.
A shaft-shaped piston rod is axially attached to one end of the piston. The piston rod is inserted through a hole in one of the closing members, and thus the piston rod is supported displaceably in the axial direction. Plural seal members are installed along the inner circumferential surface of the closing member. Liquid tightness is retained for the operation oil in the pressurizing chamber because the seal members make contact with and seal the outer circumferential surface of the piston rod, so that the outer circumferential surface of the piston rod is surrounded thereby (see, for example, Japanese Laid-Open Patent Publication No. 5-248412).
In the case of the hydraulic actuator disclosed in Japanese Laid-Open Patent Publication No. 5-248412, when operation oil is supplied to the pressurizing chamber of the main cylinder body, and the piston is displaced in the axial direction, the piston rod, which is connected to the piston, is displaced along the hole of the closing member. In this situation, the seal members installed in the hole make continuous sealing contact with the outer circumferential surface of the piston rod, and hence sliding resistance arises between the seal members and the piston rod. Therefore, for mitigating sliding resistance between the seal members and the piston rod, the seal members are lubricated with operation oil. Specifically, the operation oil, which is supplied to the pressurizing chamber, also is supplied to the hole of the closing member in which the seal members are installed.
However, the operation oil, which lubricates the seal members, originally is supplied to the pressurizing chamber in order to drive the piston. The operation oil produces optimum characteristics when the operation oil is allowed to flow through a fluid circuit (for example, an operation oil supply source and supply tube passages) to which the operation oil is supplied. As a result, the operation oil, which is supplied to the seal members, does not have characteristics optimally suitable for lubrication. Therefore, it is feared that lubrication of the seal members is insufficient, resulting in deterioration of the durability of the seal members.